Certain embodiments of the present invention relate to medical diagnostic systems, and in particular, to techniques and apparatus for adjusting the contrast of displayed diagnostic images including foreign objects.
X-ray systems are well known for creating a series of internal images of a patient, such as cardiology, radiology and fluoroscopy systems. The patient is exposed to x-rays which are then detected after passing through the patient. The radiation is pulsed to produce a continuous sequence of images which are displayed real-time on a monitor. The x-rays are attenuated as they pass through the patient. The amount of attenuation experienced by the x-rays is represented in the image by the grayscale level of the pixels that are displayed. The contrast between grayscale levels is representative of the amount of attenuation.
Bones and different types of tissues attenuate the x-rays by different amounts, and thus are detected and displayed on an image monitor with different contrast levels. For example, bone will attenuate x-ray to a larger degree than muscle and may be displayed darker than surrounding anatomy. A region of anatomy containing only soft tissue may have a smaller range of contrast than a region of anatomy containing both soft tissue and bone. In addition, scattered radiation or using an increased kVp level to image a very large patient may also decrease the contrast range.
The level of radiation detected by the system is correlated to the contrast of the displayed image by a look-up table, or transfer function. In other words, the system uses the transfer function to assign a specific level of radiation to a specific grayscale level of the display. The system varies the range of the contrast for the displayed image associated with a particular range of grayscale levels by changing the shape (e.g., slope, offset, etc.) of the transfer function. The system may have multiple transfer functions, representing different mathematical models or shapes, from which one transfer function is selected to control the contrast of one or more display images. A particular transfer function is selected for different procedures or when imaging different anatomy.
The range of contrast used to display multiple images during a procedure may be set by the system and remain constant throughout the procedure. Therefore, a level of detected radiation, or a particular brightness level, is assigned a particular grayscale level for one or an entire series of scans. This is not advantageous, as during the same procedure, areas of interest within a patient which have various ranges of contrast may be scanned, and thus the contrast may appear to fluctuate. Some images may appear with a high level of contrast, containing areas that are very dark and areas that are very light, while other images may have low contrast and appear light or washed out. An operator may chose to adjust the contrast of the displayed image to correct for the change in contrast in the anatomy, but this is time consuming, error prone, and would need to be repeated as different tissues are examined.
To provide a more constant contrast throughout a procedure (fluoroscopic, cardiology, radiology or otherwise), automatic contrast compensation algorithms have been proposed. Automatic contrast compensation, or contrast management, is utilized to present a more pleasing image with better diagnostic utility. The images are examined for maximum and minimum brightness levels as they are acquired. The maximum and minimum brightness levels are then used to determine a new grayscale transfer function to enhance the contrast of the displayed image. Therefore, a radiologist may view images that contain bone and images containing only soft tissue during the same procedure without manually adjusting the contrast.
Unfortunately, automatic contrast compensation algorithms also are sensitive to opaque materials or instruments that may be contained within the patient or may be used by radiologists, surgeons or other persons, and may appear in an image. When a foreign item such as a caliper or bone screw enters the field of view, automatic contrast compensation, when enabled, identifies a new maximum and minimum brightness level and adjusts the contrast such that the foreign item is assigned the lower end of the available grayscale, or a range of the minimum brightness levels. Therefore, the grayscale range available to display the anatomic tissue of interest is decreased, and the displayed anatomic data reflects a sudden decrease in contrast.
An operator may be able to compensate for the appearance of an instrument by disabling the auto contrast option. The transfer function may then be changed by moving it to the left or right, essentially altering the maximum and minimum values, or by changing the slope, which would increase or decrease the grayscale range. Unfortunately, if the instrument is subsequently removed from the field of view, or if the contrast of the anatomic data changes, the image would once again need to be readjusted, either by re-enabling the auto contrast option or by manually changing the transfer function. This may be time consuming and cause excessive x-ray exposure to both the patient and the radiologist or technician as the transfer function is adjusted.
Thus, a need exists in the industry for a method and apparatus for adjusting the contrast of displayed diagnostic images that include foreign objects that address the problems noted above and previously experienced.
In accordance with at least one embodiment, an x-ray system is provided utilizing an x-ray source and a detector to create x-ray data in a region of interest. The x-ray data comprises anatomic data, from the anatomy of a patient, and non-anatomic data, from a structure foreign to the patient. The anatomic and non-anatomic data have at least partially non-overlapping grayscale distribution. An input identifies a reject level along the grayscale distribution. A processor module selects a minimum value for a transfer function based in part on the reject level and automatically calculates the transfer function to convert the x-ray data to display values. A display then displays a x-ray image based on the display values.
In accordance with at least one embodiment, an automatic contrast control unit in a medical diagnostic apparatus is provided. An image processor is utilized to generate sample data of anatomic and non-anatomic structure in a region of interest. The anatomic and non-anatomic data have at least partially non-overlapping grayscale distribution. An input identifies a reject level along the grayscale distribution of the sample data. A histogram module generates a histogram of grayscale values for the anatomic and non-anatomic data. An automatic contrast module selects a minimum value for the transfer function based in part on the reject level and automatically calculates the transfer function to convert the sample data to display values.
In accordance with at least one embodiment, a method for enhancing the contrast of a diagnostic image is provided. The patient is exposed to x-rays, and then the x-ray data, comprising anatomic and non-anatomic data, is detected. The anatomic and non-anatomic data have at least partially non-overlapping grayscale distribution. A reject level located along the grayscale distribution is input. A minimum value based on the reject level is automatically calculated. A transfer function used to convert the x-ray data to display values is calculated based on the minimum value. An x-ray image is displayed based on the display values.